High frequency oscillator



NEGATIVE SUSCEPM/VCE April 2, 1957 M. s. GLASS 2, 8

HIGH FREQUENCY OSCILLATOR Filed March 4, 1954 5 Sheets-Sheet 2 POSITIVE SUSCEPTANCE INVENTOR M. S. GLASS April 2, 1957 ,M. s. GLASS 2,737,711 HIGH FREQUENCY OSCILLATOR 7 Filed March 4, 1954 :5 Sheets-Sheet 3 FIG. /0 0 FIG. ll

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flMoDE' lNl/ENTOR M. S. GLASS- I A 7' TORNE V United States Patent C HIGH FREQUEIJCY OSCILLATDR' MyronS. Glass, West. Orange, N. 33.,assignor to Bell.Tele-- phone: Laboratories, Incorporated, New'York, N. Y., a corporation of New York Application March 4, 1954, Serial No. 414,130

14- Claims. (Cl.- 250-36) This invention relates to high frequency oscillatorsand more particularly to such oscillators of the magnetron type.

A tightly coupled multi-circuitoscillating system, such as amulticavity resonator; can oscillate in a number of modes or frequencies. There is a preferred and most stable mode, referred to as the 1r-mode, in which itnormally operates as a generator. Under certain conditions.

ofi'l'o ading and driving circuits, however, a pulsed magnetron may. commence oscillation in oneof its other modes; referred'to as spurious or'd'oublet modes, thereby degrading its. performance as' a generator; This type of.- misbehavior has become known as moding and in; pulsed magnetrons causes: either partially missing or completely missing lines-in the desiredmode spectrum.

It: is known. that heavy loading of a mode of oscillation; will; retard the:buildi-up.of oscillations in that mode and. that correspondingly light loading will favor the buildup of oscillations in a mode. When a magnetron is connected toaloadiitiisevident that the principal mode of oscillation must be rather heavily loaded to extract power from the oscillator; lfa spuriousgmod'e is lightly loaded. itmay thus gain. an. advantage over the normal modeof oscillation; eventothet extent ofwexeluding the normal modefrom; a. pulsegcyclez. A; discussion of the competition that exists between: thewarious modes. at the start of the oscillation buildatp andxof moding in. general may be found in the chapter entitled: Transient. behavior by Rieke'in MicrowaveMagnetrons (Gollins, edz, MHZ Series, volume 6) and at section, 10.6 in: thear-ticle The magnetron as a. generator of centimeterwavesf byFisk, Hagstrum, and Hartman at page253, volume 25 of the Bell. System Technical. Journal: (April 1946,).

The spurious modes are doublet modes; the: two: components of whichmay, be slightly separated in; frequency;

generally the two.components.w-ill-, not-be equally loaded. When one. of. the doublet. components is heavily loaded. and the other lightly. loaded, oscillations. may build up. readily in the lightly loaded component causing; moding. It has therefore. been suggested to alleviate the modi-ng problem by assuring. equal. loading of the doubletconn ponents. of'the spurious mode nearest in frequency to the ir-l'IlOClC of the magnetron, and therefore the spurious mode in which. oscillationsare mostlikely to commence. In devices such as disclosed in. application Serial No. 348,438, filed'April 13, 1953,, of R. C. Fletcher and- S.

Millman, equal loading of-. the doublet. components of a spurious mode is attained by enlarging the diameter of specific ones of the resonator bores. In devices suchasdisclosed in application, Serial No. 348,218, filed. April 13, 1953, of J. P1 Molnar andl in application. Serial No. 348,365, filed April 13, 1953, of. M. S. Glass, this equal loading of the doublet components is assured over the.

frequency range of'a tunable magnetron byspecific variations in. the diameter of tuning pins inserted in certain of the resonator boresof the magnetron to attain tuning.

The general considerations behind the introduction. of,

reactances ofperturbations in the magnetron. circuit to attain the above stated results are set forth in detail in 2,787,711 Patented Apr. 2, 1957 "Ice 2" application. Serial No. 348,526, filed. April 13, 1953', of M. S. Glass and L. R. Walker.

Having thus attained an equalization in the loading oscillation and/ or selectively loading the degenerate mode.

of oscillation. In application Serial No. 376,022, filed July 8, 1953, of M. S. Glass, J. P. Molnar, H. G. Och,

and W. H. Thatcher there is disclosed the employment of a gaseous discharge device, of the type known. as an.

ATR tube, in the output waveguide of: the magnetron at a distance such as to present a' shortcircuit to the 1rmode ofoscillation during build up ofthe oscillation, thereby completely decoupling it from the load, While not decoupling the spurious mode of oscillation; Upon buildup of oscillation in the magnetron, the discharge device fires and the magnetron is coupled to the load. in thisprior application the spurious mode is'notadditionally loaded.

It is a general-object ofthis-invention to prevent moding of an oscillating circuit.

I-t isa further object of't'his invention to prevent moding= in-v both. tunable and single frequency magnetrons;

it: is another object of this invention to improve the operating performance of oscillatingcircuitscapable of oscillating in more than one mode, such as magnetrons. These; and other objects of. this invention are attained in; specific embodimentsin which selective resonant filters areplaced'. in the output circuit of the magnetron toprovide normal. loading for, the desired or w-mode ofoscillation: butheavier than normal loading for the undesiredspulfious modeof oscillation. Because of the short wavelength of: the, radio frequency energy generated by themagnetrons, the selective filter is placed very close to the: resonator; system. and specifically within the: vacuum envelope of the magnetron. The filter accordingly-should advantageously be very compact.

In accordance with anaspect ofrthisinvention, a waveguideirisfilter is positioned in the output waveguide of: a. magnetron but within the-vacuum envelope thereofi selectively to load the spuriouszmode closest infrequency. tothe ir-mode of oscillation. The. resonant iris; behaves as a shunt resonant circuit, resonant. at the frequency of the 1rm,ode of the magnetron; It may therefore be introduced into the output circuit of the magnetron. without afiectingthe: vr-mode loading. At the frequency. of the nearest spurious mode, however,- it introduces a; large disturbance and this disturbance is phased so as toload heavily the spurious mode. The magnitude of the dis? turbance dependson the design parameters of; the resonant iris, and; the phase in which the'reflection is preesented to the magnetron depends on the, distance. ofrthe, iris from the magnetron resonator.

Iris appreciated, that any metallic projection which partially obstructs a smooth waveguide introduces suscept: ance atthe region of. the projection, in parallel with the admittance of the guide. Familiar examplesinclude; the tuning screw and probe-type standing wave int-roducer;

An iris, consisting of a transverse septum of metal with a perforation or window, isa special type of waveguide obstacle. in accordance with one aspect of this, inventron, the characteristics. of the iris are utilized to assure the desired selective moding; Specifically it is a charac.

teristic of an iris that it. can introduce susceptance within a wide. range of values, either positive or negative. It can also be resonant at a particular frequency so that it exhibits the characteristics of a shunt resonant circuit attached. to. the line. Accordingly, if resonant at the n-Il'lOdfi frequency it introduces. no susceptance, atthe. free quency of resonance and therefore does not load the vr-mode. At other frequencies it introduces susceptance, the magnitude of which is dependent on the design of the iris and which increases with increase of deviation from the resonant frequency.

If the magnetron is tunable over a band of frequencies, a pair of resonant irises may be employed in accordance with another specific illustrative embodiment of this invention. The irises are spaced apart and spaced from the output resonator of the magnetron so that the summation of their impedances when transposed to the output resonator is represented on a Smith chart by a small curve around the characteristic impedance circle for the output line for the desired mode frequencies while providing a selective loading impedance for the degenerate mode frequencies.

It is therefore a feature of this invention that an iris be positioned in the output waveguide of an oscillator capable of oscillating in more than one mode, the iris being resonant at the desired mode frequency and spaced from the oscillator to load heavily the undesired mode of oscillation.

It is another feature of this invention that the iris be positioned from a constant frequency oscillator or an oscillator tunable over a very narrow band so as to provide maximum transposition of the impedance of the iris at the output of the oscillator for the undesired mode of oscillation.

It is a further feature of this invention that two resonant irises be employed when the oscillator is tunable over a fairly wide band of frequencies, the irises being identical and being resonant at the upper end of the desired mode frequency band.

It is a still further feature of this invention when two irises are employed that the spacing between the irises be such as to attain impedance characteristic wherein the desired mode loading is only slightly affected by the in troduction of the irises while the undesired mode loading is adversely affected by the irises. Further, in accordance with this feature, the two irises are positioned from the oscillator so that this composite adverse impedance characteristic is transposed to the output of the oscillator most effectively to load the undesired mode of oscillation.

A complete. understanding of this invention and of the above noted and other features thereof may be gained from the following detailed description and the accompanying drawing, in which:

Fig. 1 is a schematic representation, in perspective, of one specific illustrative embodiment of this invention particularly adaptable to single frequency or narrow band oscillators;

Fig. 2 is a plan view of the resonant iris employed in the embodiment of Fig. 1;

Fig. 3 is a graph of the susceptance frequency characteristic for the iris of Fig. 2;

Fig. 4 is a partial Smith chart employed in determining some of the critical factors of the embodiment of this invention depicted in Fig. 1;

Fig. 5 is a graph of circuit loading for the desired and spurious modes of oscillation of the embodiment of this invention depicted in Fig. 1;

Fig. 6 is a schematic representation, in perspective, of another specific illustrative embodiment of this invention particularly adaptable to oscillators tunable over relatively wide bands;

Fig. 7 is a plan view of an iris employed in the embodiment of Fig. 6;

Fig. 8 is a graph of the susceptance frequency characteristic for the irises of Fig. 7;

Figs. 9 and 10 are partial Smith charts employed in determining some of the critical factors of the embodiment of this invention depicted in Fig. 6;

Fig. 11 is a graph of circuit loading for the desired and spurious modes of oscillation of a tunable magnetron not incorporating this invention; and

Fig. 12 is a graph of circuit loading for the desired and spurious modes of oscillation of a tunable magnetron in accordance with the embodiment of Fig. 6.

Referring now to Fig. 1, there is depicted one specific illustrative embodiment of this invention wherein a single resonant iris is employed selectively to load the doublet components of the spurious mode nearest in frequency to the 1r-mode of oscillation. In the figure only the anode block 20 of the magnetron, the output waveguide 21, and the resonant iris 22. are depicted. The magnetron may advantageously be of the type known as the 4152, disclosed in Patent 2,466,922, issued April 12, 1949, to N. Wax.

The various critical factors that are arranged in the combination of this embodiment of the invention can best be understood from a discussion of the design procedure for determining them. These factors include the internal dimensions of the iris, the thickness of the iris, and the positioning of the iris from the magnetron output resonator. In the following discussion the frequency of the 1r-H10d6 of the magnetron is assumed to be 9375 megacycles and the frequency of the degenerate mode 10,500 megacycles, which values are approximately those for the 4152 magnetron.

The iris employed in this specific embodiment of the invention has the configuration and dimensions depicted in Fig. 2. The frequency of resonance of the iris is, of course, 9375 megacycles, and the dimensions of the slot 24 and end holes 25 can be determined readily by calculation, considering the iris as a double hole-and-slot resonator. By careful polishing out of the holes 25, to decrease the frequency, and of the slot 24, to increase the frequency, it is possible to adjust the frequency of resonance very closely to the desired value. The resonant frequency of the iris may be checked by placing it with a well-matched termination in a waveguide impedance measuring set. At resonance the iris does not disturb the match.

Fig. 3 is a graph of the line mismatch caused by inclusion of the iris 22 in the output waveguide 21 and shows the susceptance characteristic of the resonant iris. In the coordinate system of these curves susceptance is represented in terms of the standing wave it introduces in the line. This is convenient as it can be measured directly. The steepness of the slope of the line 30 determines the susceptance at neighboring frequencies. The iris depicted in Fig. 2 is particularly advantageous where the spurious mode frequency is close to the resonant frequency and therefore the curve 30 of Fig. 3 should be quite steep to assure a large susceptance at the degenerate mode frequency. High susceptance values, i. e., a steep curve 30 can be attained by employing a very narrow slot 24. Further control on the amount of susceptance introduced is attainable by varying the thickness of the metal septum. In this embodiment the iris thickness was 0.125 inch.

Fig. 4 is the skeleton of a Smith chart on which is plotted the graphic solution of the design of the iris of this specific illustrative embodiment of the invention. A Smith chart represents the complex admittance plane upon which are plotted contours of constant resistance and of constant reactance. On the chart the radial distances from the center represent reflection coefficients (or voltage standing wave ratios on a special non-linear scale) and angular distances represent lengths of transmission line. The Smith chart can be used either as an impedance chart or as an admittance chart; complete discussions of the Smith chart are known in the literature and are generally based on the article Transmission line calculator by P. H. Smith in Electronics," January 1939, pages 29 et seq.

On the skeletonized chart of Fig. 4 there is included only the circle which bounds the complex admittance plane, the diametral line of pure resistance, and the R=1 circle. This R=1 circle is the locus of impedance values 11 Efiiistive component of which is the characteristic mpedance Zn, ofthe transmission line. As only. these portions of. the. chart are needed for the explanation, of this. embodiment of the, invention, the other details. of the chart are omitted to avoid confusing lines. In the follgwing discussien, the chart is considered to be an ad- Inittance, chart so that. the R .l circle, represents, the admittance Yo of the transmission line; however, this discussion would be equally valid if the Smith chart were considered an, impedance chart for a point a. quarter wavelength. away and we can therefore speak of either the admittance or impedance characteristic depicted on the Smithcharts of Figs. 4,9. and 10.

Since the iris. 22, is very nearly free of dissipation, it will not modify. the resistive. component of the admittance f the line 21. Hence, values. of admittance resulting from. the introduction. of the iris 2.2. in. the line 21 lie along. the R=.1 circle. As, in this specific embodiment, the iris is resonant at. 9575, ru.c., the ar-mode frequency of the. magnetron 20, the introduetion of the iris does not afiect thesusceptance of the line at that frequency. Ac-. cordingly the. match. point 31, at the center of the large circle 32 on the chart, represents the admittance at that frequency. At: frequencies higher than the resonant frequency the iris 22. introduces. positive suscepta-nce in the line 21 and accordingly the, pointsfor higher frequencies definea segment 34; onthe R=1 circle 35', the segment extending to the point 36. corresponding to an admittance of 10,5QQ megacycles, which, as noted above, is approximately the. frequency of the spurious mode, nearest in he quency to the m-mode, in; this particular magnetron and therefore, the spurious mode; which; it is most advantageous t0- a ea ily.

hs oca -i n Qfi he: r u p n s on s gm t 3 and o the P int 6 a r adilybe ound. y. choosing; the de bel-va ue of h su c pt nce for the 1:18.22 from the line matchtplotof Fig, 3 and, applyingtit. to the decibel radial scale of the Smith chart, which scale is. not depictedinthe skeletonized chart of Fig. 4 The coincidence of t i ad a distanc i h: he 11:1 circle locates h po nt.

In rder'to load the spurious mode it is. necessary to determine, the, distance the iris. 22: should be placed. from the, magnetron resonator 3.8;. seen. in. Fig. 1. Advantageously, aquarter wave output transformer 39. ispositioned between; the output resonator 38; and; the output line 21, as is known in the art. Because of the length of this ransformer the susceptance variations, which. are de-. scribed; herein with. reference to the planeof: the output transformeratits. end; away. from; themagnetron, correspond; to. impedancevariations. at the output resonator itself-t. Ifdesired, therefore, the position of 'the iris'ca-n: either berdetermined fromtthe output resonator or from the pl'aneof. the output transformer. The latter determination islgenerally. made as the physical measurementof that; distance is. considerably easier.

Turning againto Fig. 4; the transposition ofthe susceptance. to theplaneof theoutput transformer is made, in terms of the Smith. chart, by rotating the segment 34 clockwise through. an. angular: distance until the point 365, corresponding to the point 36, is on the diametral line! 41. The: angular distance required forthis rotation can be read: off the Smith chart: and in this instance the distance is. (1".25-..1.8l) 7\ on 0.07X, where the wavelength is-that. of the spuriousmod'e: (10,500 mcgacycles in this embodiment). The curve 34! therefore represents the admittance which would:' be measured at a point- 0.07

closer tothe oscillator. This spacing may be increased bysone=half wavelength: to. 0.57 toavoid interference between; fringing: fields. Thus; by. positioning. the iris at this diflannc; from the end of quarter; wave transformer or substantially Q.32+n0.50-) where n; is; any integer, from; the; ol ipllt resonator the desired spurious. mode will be selectively. heavily loaded while the 1r-mode isv not lQf dcdby, the, iris. It should bepointed Qlltthatas.

the iris is. resonant at the 1r-mode frequency the iris will have. no effect on r-mode loading regardless of its position in. the waveguide. This can be seen on the Smith chart as the transposition of curve 34 comprises rotating the curve about the match point 31.

In this specific embodiment the. iris 22 was thus spaced from the end of the transformer section. 39 by a distance of 0.660 inch and was positioned within the vacuum envelope. of the magnetron 20. i

The graphs of Fig. 5 indicate the loading that can be attained this specific embodiment of the invention; These curves were taken of the specific embodiment described above comprising a. 4152 magnetron whose doublet components of the 7-mode, the mode closest in frequency to. the 1r-mode, were equallyloaded, as described above. The degree of loading in this figure is expressed as percent circuit efficiency, curve 43 being for the desired. ar-mod'e and curves 44 and 45 for the doublet 7-mode components, referred to as the 7-mode and the 7"-mode. can be seen over a narrow band of frequencies the 7--m-ode is heavily loaded, the circuit efficiency being from to 90. percent; without the iris positioned in the waveguide in accordance with our invention, the 7-mode loading would be considerably less than the 1r-mode loading, being of the order of 50 percent. An indication of the circuit efiiciency without the single. iris, can be seen from. Fig. 11, which is a plot of circuit efificiencyof modes with no output filter for a slightly different tube over substantially the same frequency band. The efficiency of the 7-mode is substantially the same for both tubes.

It. is. therefore apparent. that av single resonant iris in accordance with this invention provides a marked improvement in the loading of spurious modes for a fixed frequency oscillator or for an oscillator having a relatively narrow band of frequencies. On the graph of Fig. 5 the frequency limits specified for the 4152 magnetron are depicted to illustrate this point.

Curves, 44 and 45- are indicated by broken lines over their high efiiciency portions. because it is very difficult accurately to measure suchhigh. circuit efiiciencies and there is. accordingly a slight degree of uncertainty in the measurementsa-t these points. Despite these quantitative uncertainties, however, it is apparent thatv over a band considerably wider than that specified for this particular magnetron, the spurious mode closest in frequency to the 1r-mode, is: much more heavily.- loaded than the 1r.IIl0d6..

While the embodiment of this invention depicted in Fig, l and described above is usable only over. a relatively narrow frequencyv band, some advantage. may be attained in. accordancefwith another specific illustrative embodiment of this invention employing a pair of resonant irises when. the oscillator. is tunable over a relatively wide band of frequencies. Turning now to Fig. 6 there is depicted an. embodiment of this invention employable when. the magnetron 50 is tunable over a wide band of frequencies. type. disclosed in Patent 2,657,334 issued October 27, 1953, to J. W. West. An outputv waveguide 51 is connected by a quarter-wave transformer section 52 to the output resonator. bore 53. Positioned in the waveguide 51, advantageously within the vacuum envelope of the magnetron, are a pair of identical resonant irises 55 and 56inv accordancewiththis invention. The shape and dimensions of the irises 55 and 56 for one specific embodiment are shown in Fig. 7, the irises in this embodiment being. 0.062 inch thick.

As before an appreciation of the critical parameters and positioning of. the irises in accordance with this embodiment can best be understood from a discussion of the design procedure for one specific example. In this example a magnetron of the type disclosed in the abovementioned West patent is assumed, having a 1r.-mode fre qucncy-bandiof from- 8500'. to 9600 megacycles. The

The magnetron 50 may be of the.

7 graph of the line mismatch for the iris depicted in Fig. 7 is shown in Fig. 8 and is represented by the line 59. Figs. 9 and 10 are skeletonized Smith charts of the type described above with reference to Fig. 4.

Turning now to Fig. 9, there are plotted the admittances of the two irises 55 and 56 at the plane of iris 55. Curve 61, which is a segment of the R=1 circle, is the admittance of the iris 55 nearest the oscillator 59 for a band of frequencies from 8500 to 10,600 magacycles. This band of frequencies covers the frequency range of both the 1rmode and the nearest spurious mode. The match point 62 is determined by the resonant frequency of the irises 55 and 56 and in accordance with an aspect of this invention is the upper limit of the 1r-mode frequency band. In this embodiment the magnetron is tunable over a range of from 8,500 to 9,600 megacycles and therefore the match point is at 9,600 megacycles.

The admittance characteristic of the more remote iris 56 is identical with that of iris 55 and, in its own plane, would appear as curve 61. However, that characteristic must be transposed to the plane of the closer iris 55 and is therefore rotated through an angular distance which corresponds to the distance between the irises, as shown by the arrowed-line 64. The rotated characteristic for iris 56 is curve 65 in Fig. 9. When the admittances of the two irises are thus located in the same transverse plane of the waveguide 51, they are effectively in parallel across the line at that plane. Thus they set up a resultant admittance which is the algebraic sum of the admittances due to the individual irises.

Accordingly by adding each point on curve 65 to the point of that frequency on curve 61, the curve 68 corresponding to the actual admittance of the irises 55 and 56 at the plane of the iris 55 is determined. By looking at this curve 63 there can readily be observed the desiderata which determine the spacing between the irises 55 and 56. The 1r-mode frequency range is from the lower frequency end of curve 68 to the match point 62 and, in this embodiment, is from 8500 to 9600 megacycles. The spacing between the irises should be such that, when the admittances are added together, the composite admittances in this range are points on curve 68 closely surrounding the match point, which is the point of no loading. As can be seen in Fig. 9 the curve 68 has a small loop adjacent the match point, which loop includes the entire vr-mode frequency band. Further, as will be apparent from Fig. 10, the low frequency point 69 and the farthest point of this loop should be approximately on a straight line. The spacing between the irises 55 and 56 is therefore chosen so that when the transposed admittance of the iris 56 is added to the admittance of iris 55 the resultant admittance curve has these Characteristics.

The distance of the iris 55 from the plane of the output transformer 52 is also determined by the angular distance required to transpose the admittance curve 68 along the arrowed-line 73 so that the high frequency point 71 of curve 68 lies on the diametral line. As pointed out above with reference to the prior embodiment, this admittance is transposed by the quarter-wave transformer matching section 52 to an impedance characteristic having the same relative distribution of values at the output resonator 53 of the magnetron 50. Thus the relative loading of the magnetron at various frequencies in the band may be deduced directly from the characteristic depicted by curve 68, considering curve 68 to be either an admittance characteristic at the output of the transformer 52, or an impedance characteristic at the magnetron itself.

Referring again to curve 68 it can be noted that at the lower end 70' of the rr-mode frequency range the loading is outside the R=1 circle and is therefore somewhat lighter than normal. It rather quickly drops, as the frequency increases, to the match point 62 and then over the remainder of the vr-mOdB range it forms a closed loop closely hugging the R 1 circle, which circle represents 8 normal resistive loading of the magnetron. When the frequency goes above match point and therefore above the upper limit of the vr-mode frequency range, the impedance circle swings into the region of heavy loading inside the R=l circle. The spacing between the two irises and 56 is therefore determined to have this closed loop, when transposed to the magnetron output, along the 12:1 circle.

It is therefore apparent that by employing a pair of irises resonant at the upper frequency of the rr-mode range and separated from the output resonator of the magnetron and from each other in accordance with aspects of this invention, the 1r-1I10de loading of the magnetron stays fairly constant over the 1r-1'l'lOde frequency range except for a small unloading effect at the low frequency end. This effect at the low frequency end may be useful in flattening the output power characteristic. It is also apparent that if the frequency range and intrinsic mode separation of the oscillator is such that the low frequency end of the spurious mode range very nearly coincides with the high frequency end of the 1r-mode range very little improvement in spurious mode loading can be expected in that region. However, for higher spurious mode frequencies the spurious mode loading is substantially increased, as is apparent from Fig. 10.

In this specific embodiment, the spacing between the irises 55 and 56 was 0.440 inch and the iris 55 was spaced from the end of the output transformer section 52 by a distance of 0.160 inch. The particular iris configuration employed, readily seen in Fig. 7, has a relatively low slope susceptance curve which is desirable when the separation between the mode frequency ranges is very slight, as discussed further below. The resonant frequency of the iris can be readily determined by variations in the dimensions of the rectangular aperture in the iris, an increase in a major dimension lowering the frequency of resonance and an increase in a minor dimension raising the fre quency of resonance.

The improvement attained by this invention can be seen in Figs. 11 and 12 wherein a magnetron of the type disclosed in the above mentioned West patent was tunable over the wide range of frequencies from 8500 to 9600 megacycles and the closest spurious mode, the 7-mode, had its lower end almost coinciding with the upper frequency end of the w-mode. In Fig. 11 there is depicted the amount of loading, expressed in terms of circuit efficiency, over the 1r-mode frequency band of the doublet 7-mode components, identified as the 7 and 7" modes, and the 1r-mode in a resonator structure which has been modified to equalize the loading of the 7-mode components, as described above. In Fig. 12 there is depicted the same curves for the magnetron after a suitable double resonant iris filter has been inserted in the output, in accordance with this invention. It will be noted in Fig. 12 that while the 7-mode circuit efiiciency has not been increased much at the low frequency end of the band, the wr-mode coupling has been reduced so that the relation between the two has become more favorable to the starting of the 1r-mode oscillations. The loading of the 7-modes becomes greater as the frequency is increased so that at the high frequency end of the band they are loaded more heavily than the rr-mOdC. While this increase in loading is modest, as compared to that which may be attained in a'single frequency or narrow frequency band magnetron, its effect on performance is nevertheless considerable. Further these curves of Figs. 11 and 12 are for the most adverse case where there is substantially no frequency separation between the Tr-mode frequency range and the 7-mode frequency range. In other embodiments wherein there is a greater separation of mode frequencies inherent in the oscillator itself, the selective increase in loading of the spurious modes is of course considerably greater.

It is to be understood that the above described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In combination, a generator of ultra-high frequency capable of oscillating in more than one mode, output means including an output waveguide connected to said generator, and means for assuring build-up of oscillation in said generator in a preferred mode, said last-mentioned means including an iris filter positioned transversely in said Waveguide, said iris filter being resonant at a frequency of said desired preferred mode and positioned from said generator a predetermined, distinct electrical distance so as to load a spurious mode of oscillation.

2. In combination, a magnetron capable of oscillating in more thanone mode and having a plurality of cavity resonators, an output waveguide coupled to one of said resonators, and an iris filter positioned in and across said waveguide, resonant at a ir-mode frequency of the magnetron and positioned from said one resonator a predetermined, distinct electrical distance so as to load a spurious mode of oscillation.

3. In combination, a magnetron for generating power in a restricted frequency range but capable of oscillating in more than one mode, an output waveguide coupled to said magnetron, and an iris filter positioned transversely in said Waveguide, resonant in said restricted frequency range, and positioned from said magnetron a predetermined, distinct electrical distance so as to load heavily a spurious mode of oscillation.

4. In combination, a generator of ultra high frequency power in a restricted frequency range but capable of oscillating in more than one mode, an output waveguide coupled to said generator, and means for assuring buildup of oscillation in a preferred mode, said means including an iris filter positioned transversely in said waveguide and dimensioned to be resonant in said restricted frequency range, said iris being spaced from said generator a predetermined, distinct electrical distance so as to load heavily a spurious mode of oscillation.

5. In the combination of claim 4, said iris being spaced from said generator an electrical distance so that the impedance characteristic of said iris on a Smith chart when rotated through said electrical distance has the point on said characteristic corresponding to said spurious mode fgequency transposed to the diametral line of said Smith c art.

6. In combination, a generator of ultra-high frequency power over a relatively wide frequency range and capable of oscillating in more than one mode, an output waveguide coupled to said generator, and means for assuring build-up of oscillation in a preferred mode, said means including a pair of iris filters positioned in and across said waveguide and dimensioned to be resonant in said frequency range, said irises being spaced from each other and from said generator so that the impedance of said irises transposed to said generator loads a spurious mode of oscillation but has substantially little effect in loading a desired mode of oscillation.

7. In the combination of claim 6, said irises being identical and being resonant at the upper frequency of said frequency range.

8. In combination, a magnetron tunable over a relatively wide frequency band and capable of oscillating in more than one mode, an output waveguide coupled to said magnetron, and a pair of iris filters positioned in and across said waveguide, each dimensioned so as to be resonant at the upper frequency of said band, and positioned from said magnetron so as effectively to load a spurious mode of oscillation while having substantially little effect on the desired 1r-mode of oscillation.

9. in combination, a magnetron, an output Waveguide coupled to said magnetron, and means positioned in said waveguide to prevent moding of said magnetron, said means comprising a transverse metallic plate defining an iris filter, said iris filter being resonant at the vr-mode frequency of said magnetron and being spaced therefrom a predetermined, distinct electrical distance so that the impedance of said iris at a spurious mode frequency is transposed to said magnetron heavily to load said spurious mode.

10. in the combination of claim 9, said transverse metallic plate having a pair of apertures therein connected by a slot, said slot being narrow and said plate having a thickness so that said iris defined thereby has a steep susceptance-frequency characteristic.

11. In the combination of claim 9, said iris being spaced from the output of said magnetron an electrical distance such that the impedance characteristic of said iris on a Smith chart when rotated through said electrical distance has the spurious mode point on said characteristic transposed to the diametral line of the Smith chart.

12. In combination, a magnetron tunable over a band of frequencies, an output waveguide coupled to said magnetron, and means positioned in said waveguide to prevent moding of said magnetron, said means comprising a pair of transverse metallic plates defining iris filters resonant at the upper frequency of said band of frequencies of said magnetron and spaced from said magnetron an electrical distance so that the composite impedance of said irises when transposed to said magnetron selectively loads a spurious mode of oscillation over said band of frequencies.

13. In the combination of claim 12, said metallic plates being identical and each having a substantially rectangular aperture therein, the thickness of said plates being such that said irises have relatively slowly rising susceptancefrequency characteristics.

14. In the combination of claim 12, said irises being spaced from each other so that the sum of the impedance characteristic of the more remote iris and the impedance characteristic of the iris nearer the magnetron at the plane of the nearer iris is a curve on the Smith chart adjacent the match-point of the chart over the desired band of frequencies and removed therefrom over the band of frequencies of a spurious mode and said nearer iris being spaced from said magnetron such that said curve is rotated on said Smith chart so that the portion defining the spurious mode frequencies is within the R=l circle and the portion defining the desired mode frequencies is substantially along the R=1 circle at the plane of the output of said magnetron.

References Cited in the file of this patent UNITED STATES PATENTS 2,523,841 Nordsieck Sept. 26, 1950 2,582,205 Longacre Jan. 8, 1952 2,644,889 Finke et al. July 7, 1953 

